(1) Field of the Invention
The present invention relates to vehicle control systems and methods and more specifically to systems and methods for controlling a trajectory of a supercavitating vehicle.
(2) Description of the Prior Art
There exists a need for reducing drag in underwater vehicles, such as torpedoes, so as to enhance their speed, reliability and stealthy operation. For a nominally streamlined, fully wetted underwater vehicle, 80% of the overall drag is skin friction drag. The remaining drag is pressure or blockage drag.
Some investigations into reducing the drag of high-speed, underwater vehicles have focused attention on supercavitating underwater vehicles. For supercavitation, sufficient energy is put into the water to vaporize a given volume of water to form a cavity through which the object travels. When a vapor cavity completely encapsulates an underwater object, the process is referred to as supercavitation. Supercavitation allows for the higher speeds to be sustainable by reducing skin friction drag to a great extent at such higher speeds. The conditions for supercavitation are known in the art.
To obtain supercavitation, fluid may be accelerated over a sharp edge of the vehicle so that the pressure in the fluid drops below its vapor pressure after passing the edge. The component resulting in the pressure drop may be referred to herein as a cavitator. The cavitator, generally part of the nose shape of the object, is the only part of the object in constant contact with the water through which the vehicle travels.
However, if the speed of the vehicle is not sufficiently fast, the vapor cavity may collapse about the trailing portions of the vehicle. In such cases, artificial ventilation may be introduced into the cavity, which maintains the cavity beyond the trailing edge of the object.
In a ventilated cavity, i.e., one maintained by vaporous or artificial cavitation, the stability of the cavity interface can be maintained by insuring that the gas within the cavity is moving at same the speed as the vehicle. This reduces instabilities at the air-water interface. As a result the vehicle within the cavity is surrounded by high speed ventilation gas.
For stability and control, current supercavitating vehicles rely on one or more tail fins extending radially from an aft portion of the vehicle. Stability is maintained by tail planing, whereby the tail fin is extended into the air-water interface of the cavity. Control surfaces on the tail fins control the vehicle trajectory.
However, the existing system of aft vehicle control produces considerable parasitic drag. In addition, such systems create continuous low frequency oscillations that result in intermittent banging of the tail surface on the air-water interface. The banging results in structural vibration, which increases the noise emanating from the vehicle and thereby decreases the overall stealth of the vehicle.
What is needed is a system for efficiently controlling a supercavitating vehicle. The system should provide an active damping mechanism such that a vehicle riding in an underwater supercavity needs not tail-plane. The system should reduce both the streamwise drag of the vehicle and the structural vibration induced by tail-planing, thus enhancing the stability of the vehicle, increasing its range and improving its stealth. In addition to stabilizing the vehicle in steady flight, the system should provide for quickly turning the vehicle as well.